1. Field of the Invention
This invention relates to crystalline material for use in electro-optic devices and particularly to such material formed by epitaxially growing successive layers of silver thiogallate crystals from a solution including a dopant to form layers having different resistivities.
2. Description of the Prior Art
Certain electro-optic tunable filters, modulators, switches, shutters, and resonators are electronically tunable. In order to make an electronically tunable device, such as a tunable optic filter, a spatially varying dc electric field is applied to successive regions of a birefringent crystalline medium disposed between a pair of suitably oriented polarizers along the path of a light beam to be filtered. The dc electric field may be either parallel or perpendicular to the light path. This field electro-optically produces in the birefringent medium an effective oscillation of the optic axis of the medium about the pass direction of the input polarizer. The oscillation is in a plane perpendicular to the light path as a function of distance along the light path, with a predetermined spatial variation to achieve a desired filter transmission characteristic.
The potential use of silver thiogallate (AgGaS.sub.2) in tunable filters and other electo-optical devices is described by Gopal C. Bhar and R. C. Smith in an article "Silver Thiogallate (AgGaS.sub.2) - Part II: Linear Optical Properties, IEEE Journal of Quantum Electronics, Vol. QE-10, No. 7 (July 1974) pp. 546-550. In this article, it is pointed out that the remaining problems, which have prevented utilization of AgGaS.sub.2 in infrared nonlinear optical devices, are primarily in the area of development of crystal growth methods which would provide a crystal free from twinning defects and residual absorption due to scattering from second phase precipitates.
Many of the devices in which use of the AgGaS.sub.2 crystal would be most advantageous require multiple layers of crystal, with each layer varying in composition so that the entire structure can be tuned electronically to provide the desired electo-optical characteristics. This has been accomplished using a number of individual layers of single crystal plates bonded together into a stack arrangement as described in U.S. Pat. No. 4,197,008, "Electro-Optic Tunable Optical Filter", U.S. Pat. No. 4,229,073 "Iso-Index Coupled-Wave Electro-Optic Filters", U.S. Pat. No. 4,350,413, "Multi-Color Tunable Filter", and U.S. Pat. No. 4,240,696, "Multilayer Electro-Optically Tunable Filter". Further details concerning use of semiconductor crystals in such devices may be found in J. P. Laurenti et al, "Optical Filters Using Coupled Light Waves in Mixed Crystals", Applied Physics Letters, Vol. 28, No. 4 (Feb. 15, 1976) pp. 212-213 and J. P. Laurenti et al, "Graded-Composition Semiconductors as Tunable Narrow-Band Optical Filters", Journal of Applied Physics, Vol. 48, No. 1 (January 1977) pp. 203-204.
However, the bonding of a number of individual layers results in handling and assembly problems encountered when thin layers of crystal are sliced from the bulk-grown crystals and polished and bonded together to form the multiple layer structures.
U.S. Pat. No. 4,535,822, which issued as a patent on Aug. 13, 1985, by S. R. Sashital entitled, "A Method of Synthesizing Highly Pure, Thin, Single Crystal of Silver Thiogallate", filed on the same date as this application, discusses the use of silver thiogallate as a suitable crystal material for electro-optic devices. Herein this patent application will be referred to as the Sashital I Application. In the Sashital I Application there is disclosed a method of synthesizing a single crystal layer of silver thiogallate by growing it epitaxially on a seed crystal from a molten solution of silver thiogallate in either antimony sulfide or lead sulfide. This crystal layer alone, however, is not suitable for the electrical optic device of the type discussed herein.
U.S. Pat. No. 4,540,461, which issued as a patent on Sept. 10, 1985, by S. R. Sashital, R. L. Joyce, and A. L. Gentile, entitled, "Silver Thiogallate Single Crystal Epitaxial Grown from Potassium Chloride-Silver Thiogallate Solution", filed the same day as this application, discloses an improvement in the method of the Sashital I Application. Herein this patent application of Sashital et al will be referred to as the Sashital II Application. In this Sashital II Application there is disclosed the use of potassium chloride as a solvent for the silver thiogallate, replacing the antimony sulfide or lead sulfide. According to the invention of the Sashital II Application, a single crystal layer of silver thiogallate is grown on a seed crystal substrate of, preferably, silver thiogallate crystal. First, a molten solution of silver thiogallate and potassium chloride is prepared and then the seed crystal substrate is dipped into this molten solution and the solution's temperature slowly lowered. The reduction in temperature forms on the seed crystal substrate the layer of single crystal silver thiogallate. The problem nevertheless remains of producing a multilayered material comprising layers of single crystals bonded together and having a conducting member with a suitably matched refractive index disposed between adjacent crystal layers which does not degrade the light signal passing through the device. The present invention overcomes this problem.